The heart of a computer is a magnetic hard disk drive (HDD) which typically includes a rotating magnetic disk, a slider that has read and write heads, a suspension arm above the rotating disk and an actuator arm that swings the suspension arm to place the read and/or write heads over selected circular tracks on the rotating disk. The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk adjacent an air bearing surface (ABS) of the slider causing the slider to ride on an air bearing a slight distance from the surface of the rotating disk. When the slider rides on the air bearing the write and read heads are employed for writing magnetic impressions to and reading magnetic signal fields from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
The volume of information processing in the information age is increasing rapidly. In particular, it is desired that HDDs be able to store more information in their limited area and volume. A technical approach to this desire is to increase the capacity by increasing the recording density of the HDD. To achieve higher recording density, further miniaturization of recording bits is effective, which in turn typically requires the design of smaller and smaller components.
The further miniaturization of the various components, however, presents its own set of challenges and obstacles. Finer ferromagnetic crystal grains in recording media and noise reduction are effective ways to raise the recording density. However, as the size of the crystal grains decreases, the recording magnetization becomes thermally unstable, and thermal demagnetization, e.g., degradation, arises in the output signal and the read/write characteristics of the magnetic storage device deteriorate over time. An effective method to prevent thermal demagnetization is to increase the magneto-crystalline anisotropy of the ferromagnetic crystal grains, but a large magnetic field is required at the magnetic head to enable recording on a medium having large magneto-crystalline anisotropy.
A narrower track width in the magnetic head is also effective to achieve high recording density. However, as the track width in the magnetic head becomes narrower, the recording magnetic field of the magnetic head becomes smaller. This phenomenon is referred to as the “trilemma” of high recording densities and hinders the development of high recording densities when using conventional technologies.
Microwave-assisted magnetic recording (MAMR) is capable of recording to a medium having high magneto-crystalline anisotropy, and may be used to overcome this trilemma. MAMR incorporates a microwave magnetic field oscillation element into the recording head and records by superimposing a microwave magnetic field on the recording magnetic field of the head. Ferro-magnetic resonance (FMR) occurs when the frequency of the microwave magnetic field matches the resonance frequency of the magnetization of the medium and the spin precession is further activated. This FMR is able to lower the energy barrier to magnetization reversal. An element referred to as a spin torque oscillator (STO) that is separated from ferromagnetic thin films by non-magnetic layers may be used as a microwave magnetic field oscillation element. This structure, when placed between the main magnetic pole and the trailing shields, forms a MAMR recording head.
Because FMR is used in MAMR, it is important to appropriately control the magnetic characteristics of the medium such that they correspond to the frequencies of the microwave magnetic field in order to obtain a high assist effect. For example, Japanese Patent Office (JPO) Patent No. 4960319 proposes a magnetic recording device provided with a magnetic recording head that has a main magnetic pole and a spin torque oscillator arranged close to the main magnetic pole and includes at least two magnetic layers of a spin injection layer and an oscillation layer, and a magnetic recording medium that includes the two magnetic layers of a recording layer and an antenna layer, where at least the recording layer is a hard magnetic material, the antenna layer is formed at a position closer to the magnetic recording head than the recording layer, the resonance frequency, fa, of the antenna layer is lower than the resonance frequency, fr, of the recording layer, the recording layer and the antenna layer are ferromagnetically coupled, and the resonance frequency of the antenna layer is larger than the resonance frequency of the oscillation layer. This is capable of obtaining a high assist effect by using a microwave magnetic field.
However, it is not sufficient to only implement a high assist effect due to the microwave magnetic field in order to use MAMR and achieve a high recording density. If the magnetic characteristics of the medium are not appropriately controlled to correspond to the recording magnetic field of the head, satisfactory recording characteristics will not be obtained even if a high assist effect is obtained.